1. Technical Field
The present invention relates to an ultrasonic transducer and a method of manufacturing the ultrasonic transducer. In particular, it relates to an electrostatic ultrasonic transducer capable of increasing the efficiency of conversion between an electrical signal and a sound signal to increase an output sound pressure level and facilitating micromachining of a fixed electrode (a lower electrode) necessary therefor and a method of manufacturing the electrostatic ultrasonic transducer.
2. Related Art
Most related-art ultrasonic transducers are of a resonance type that use piezoelectric ceramics. A structural example of the related-art resonant ultrasonic transducers is shown in FIG. 10. The ultrasonic transducer shown in FIG. 10 performs both conversion from an electrical signal into an ultrasonic wave and conversion from an ultrasonic wave into an electrical signal (transmission and reception of an ultrasonic wave) using piezoelectric ceramics as a vibration element.
The bimorph ultrasonic transducer shown in FIG. 10 includes two piezoelectric ceramics 61 and 62, a cone 63, a case 64, leads 65 and 66, and a screen 67. The piezoelectric ceramics 61 and 62 are bonded together. The leads 65 and 66 are connected to the surfaces opposite to the bonded surfaces, respectively. The resonant ultrasonic transducer uses the resonance phenomenon of the piezoelectric ceramics, so that ultrasonic transmission- and reception-characteristics are in good condition in a relatively narrow frequency band around its resonant frequency.
Unlike the resonant ultrasonic transducer shown in FIG. 10, electrostatic ultrasonic transducers have been known as broad-band-oscillating ultrasonic transducers capable of generating high sound pressure across a high frequency band. A concrete example of the broad-band-oscillating ultrasonic transducers is shown in FIGS. 11A and 11B.
The electrostatic ultrasonic transducer shown in FIG. 11A uses a dielectric (an insulator) 131 of the order of 3 to 10 μm thick, made of polyethylene terephthalate resin, as a diaphragm or vibrator. To the dielectric 131, an upper electrode 132 made of metal foil such as aluminum is formed on the upper surface thereof by vapor deposition, and a fixed electrode (a lower electrode) 133 made of brass or the like is disposed below the lower surface of the dielectric 131. The dielectric 131 and the upper electrode 132 (Bank of the upper electrode 132) contact each other by applying a DC bias voltage, and the dielectric 131 vibrates by applying an AC voltage.
Random uneven microscopic asperities of the order of tens to several hundred μm are formed on the surface of the lower electrode 133 adjacent to the dielectric 131. The asperities form a space between the lower electrode 133 and the dielectric 131, so that the distribution of the capacitance between the upper electrode 132 and the lower electrode 133 varies slightly. The random microscopic asperities can be formed by roughening the surface of the lower electrode 133 manually. The electrostatic ultrasonic transducer has such asperities to form a large number of capacitors. Accordingly, the frequency response of an ultrasonic transducer 122 can be a broad band response as shown by curve Q1 in FIG. 11B. The asperities also offer the advantage of increasing the efficiency of conversion between an electrical signal and a sound signal (increasing the level of output sound pressure).
The characteristics of the electrostatic ultrasonic transducer are thus improved by the asperities on the fixed electrode. However, when the surfaces (the upper surfaces) of the projections of the asperities are flat, a strong electrostatic force is applied to the space between it and the diaphragm to make the diaphragm stick to the flat surfaces of the projections, so that the diaphragm is sometimes restrained by the projections and so hardly vibrates, decreasing the efficiency of conversion between a sound signal and an electrical signal.
Several inventions of an electrostatic ultrasonic transducer have been disclosed in which asperities (voids) are provided between the diaphragm and the fixed electrode (the lower electrode). For example, an invention in which voids are formed by a dielectric spacer is disclosed in JP-A-2000-50392 and an invention in which the fixed electrode is provided with communication holes connecting with the groove, thereby decreasing the resonant frequency and increasing the conversion efficiency between a sound signal and an electrical signal is disclosed in JP-A-58-46800.
However, the related arts have not been able to solve the problem of the diaphragm sticking to the flat surfaces of the projections so that the diaphragm is restrained by the projections and hardly vibrates.